Manufacturing method of a tyre tread

ABSTRACT

A method of manufacturing an agricultural tyre (1) with deep cross-lugs (6), comprising the steps of: (a) proving a tyre case (2) and tread belts (3); (b) applying rubber blocks (206) onto tread belts (3), each of such rubber blocks (206) being arranged to make a respective lug (6) of the final tread (4); (c) winding a rubber ribbon (200) above said rubber blocks (206) so as to completely enclose said rubber blocks (206); (d) curing the rubber blocks (206) together with the wound ribbon (200) in a curing mould (105) negatively reproducing a desired pattern of the tread (4).

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and a system for manufacturinga tyre tread having a deep tread pattern, in particular a tread withlugs.

The invention pertains mainly to all the tyres featured of lug treadpattern, in particular to agricultural tyres and, generally speaking, totyre used on off-road work vehicles, for example farm vehicles,earthmoving machinery and the like.

BACKGROUND

Heavy-duty tyres of the type mentioned above have a tread with atoroidal base extending about a central axis of rotation of the tyre,from which base a plurality of (deep) lugs project radially outwardly.Each lug extends substantially crosswise to the travelling direction ofthe tyre and at selected angles with respect to adjacent lugs.

At present, such tyres with lugs are manufactured according to thefollowing steps. A tyre casing is prepared and arranged onto a buildingdrum. Tread belts, made of metal or textile, are wound about the casingand a smooth, green-rubber blank tread is wound over the belts. Thisassembly is inserted inside a curing mould negatively reproducing thetread (i.e. lugs) pattern. During the curing process, the rubber of theblank tread is heated and subjected to high pressure to adapt to theshape of the curing mould and so form the tread pattern.

As mentioned above, the tread pattern includes lugs which may beexceptionally large and may project upwards considerably from the restof the tread. Accordingly, forming the lugs ex novo in the curing mouldinvolves considerable rubber displacement inside the mould, i.e. a largeamount of rubber must migrate from one part of the mould to another.

This rubber flow inside the curing mould prevents the formation of anevenly thick tread base. Such uneven tread base (differing widely inthickness) may result in defects and flaws.

Moreover, in order to prevent too thin a rubber thickness at thethinnest parts of the tread base, the average thickness of the treadbase must be ‘oversized’ with respect to an optimum thickness that couldbe used if the tread base were perfectly even. Oversizing the averagethickness of the tread base means that more material (rubber) isrequired to produce the tyre (with no advantage in terms of nominal tyreperformance), thus increasing the manufacturing costs and the tyreweight.

To eliminate these drawbacks, it has been proposed to reduce thethickness of the blank tread wound about the casing (with the belts inbetween) and to apply green-rubber blocks, or bars, at the areas of theblank tread where the lugs are to be formed. This arrangement reducesthe amount of rubber that has to migrate from one part of the curingmould to another, by providing more rubber (in the form of rubberblocks) where it is needed (to form the lugs) right from the onset. Evenusing rubber blocks, however, the thickness of the tread base may stillremain significantly uneven, especially at tyre sides, or shoulders,where the lugs are generally larger.

Improvements to this known art method aimed at reducing the thickness ofthe tread base and its unevenness following the moulding process havebeen proposed in WO2013156982A1 and WO2013156983A1.

Notwithstanding the above, the known art methods to manufacture tyreswith lugs are still perfectible in several aspects.

First of all, an important risk remains that, during tyre service on thefield, separation or cracking does occur at the connection, or junction,between the base tread and the protruding lugs. This inconvenience isrelated also to the fact that the junction between each block (i.e. lug)and tread base is a “cold” one, being generally obtained duringvulcanization (and not by extrusion). Said separation or cracking mayaffect block (i.e. lug) junctions at both the tyre centre and shoulders.Moreover, even when the manufacturing methods provides that blocks areapplied onto a tread blank, moulding defects relating to rubber flow maystill happen. Furthermore, the quantity of rubber material used and,consequently, tyre weight are still to be optimized.

SUMMARY OF THE INVENTION

The technical problem underlying the present invention is therefore toprovide a method of manufacturing a tyre, or a tyre tread, with lugswhich allows overcoming the drawbacks explained above with reference tothe known art.

The above problem is solved by a manufacturing method according to claim1 and by a corresponding manufacturing apparatus according to claim 10.

Preferred features of the invention are recited in the dependent claims.

The method of the invention provides arranging shaped green blocks, orbars, above tread ply or plies (or belt/belts) and winding a shapedrubber ribbon on the green blocks. Subsequently, moulding can beperformed in a conventional moulds.

In this way, by curing in the mould, the rubber ribbon makes the treadbase and the combination of each block with a corresponding portion ofthe rubber ribbon makes a respective lug.

Blocks can be placed above tread plies onto a known drum of a TyreAssembly Machine (TAM) off-line with respect to the manufacturing cycle,onto a TAM server or even off-line in a dedicated station.

The tread ribbon can be applied on line at TAM, with no need to extrudeit off-line and then transport to TAM.

Generally speaking, the method does not require for any tread blank tobe applied below the bars or blocks, i.e. directly onto the tread plies.

However, variant applications do not exclude that additional rubbermaterial can be provided below the bars.

The method of the invention eliminates the risk of openings, cracking orseparations at the junction between blocks (lugs) and tread base duringservice, as well as bar junctions contamination. In fact, the barjunctions are inside, and enclosed by, the ribbon and therefore areprotected and not in contact with the soil. In other words, the bars areinside the carcass and the junctions are protected by the rubbermaterial of the ribbon.

Similarly, the tyre configuration according to the invention allowspainting the green tire outside (e.g. by anti-blemish paint, releaseagent, or others) with no risk of bar junction contamination.

Moreover, bars can have a thinner gauge, so that they are easier toprocess (in particular during operations of extrusion, winding orcutting) and also easier to assembly on tread plies.

The invention also allows reducing rubber flow into lug parts of themould and avoiding excess of rubber not needed to fill lug volume.

The invention is also capable of reducing rubber at inter-lug skid basearea.

Tread weight can also be reduced, thanks to excess of rubberelimination, particularly at shoulder areas.

Furthermore, the invention allows producing tread ribbon and bars ofdifferent compounds. Typically, the cost of the bars compound can bereduced, as it is shielded by the tread ribbon. Moreover, the inventionallows producing bar and/or tread ribbon by recycled rubber.

Additional advantages are also die shape communality among differenttyre sizes and models, improvement in tyre moulding defects (both atcentre and shoulders) and a lower risk of trapped air between lugs andtread base.

Other advantages, features and use modes of the present invention willbe understood from the following detailed description of someembodiments, provided by way of example and not with limitative purpose.

SHORT DESCRIPTION OF THE DRAWINGS

Reference will be made to the figures of the annexed drawing, wherein:

FIGS. 1 and 2 relate to an exemplary agricultural tyre which can bemanufactured by a preferred embodiment of the method according thepresent invention, such figures showing a perspective front view and apartial cross-sectional view according to an axial plane of said tyre,respectively;

FIG. 3 shows a schematic representation of a sequence of manufacturingsteps according to the aforementioned preferred embodiment of themethod;

FIG. 3A shows a perspective view of a block or bar used in the method ofFIG. 3;

FIG. 3B shows a perspective frontal view of an intermediate producthaving a tyre case, tyre belts or plies and bars or blocks appliedthereon according to the method of FIG. 3, which intermediate product isbeing formed onto a drum;

FIG. 3C shows a perspective frontal view of a subsequent intermediateproduct having also a tread ribbon applied onto the bars according tothe method of FIG. 3;

FIG. 3D shows a perspective frontal view of a final intermediate productready for a moulding step according to the method of FIG. 3; and

FIG. 4 shows a preferred embodiment of an exemplary cross-sectionalprofile of the bar of FIG. 3A.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

With reference to FIGS. 1 and 2, an agricultural tyre 1 has a centralaxis of rotation A. A rolling direction of tyre 1 is denoted by 7.

Tyre 1 comprises a toroidal casing 2, which extends about a central axisof rotation A and supports tread belts, or plies, 3 wound about casing2.

Tyre 1 comprises a tread, globally denoted by 4, arranged about casing 2and over belts 3 according to a toroidal configuration. Tread 4comprises a tread base 5, which completely covers casing 2 and belts 3,and a plurality of lugs 6. Lugs 6 project upwards (i.e. extend radiallyoutwards) from tread base 5 and are arranged substantially crosswisewith respect to a tyre travelling direction.

Typically, lugs 6 are arranged symmetrically about axis of rotation A.

In the present embodiment, each lug 6 curves from a centreline C of tyre1 to a corresponding shoulder and has a cross-sectional shape taperingaway from tread base 5.

Each lug 6 has a lateral portion 8 at a shoulder of tyre 1 and a centralportion 9 located in the central area of tyre 1. In the presentembodiment, each lug 6 is thinner at central portion 9 and thicker atlateral portion 8. In the example provided, central portion 9 of eachlug 6 has a greater curvature than lateral portion 8 (lateral portion 8may also be assumed to be substantially straight).

As will be explained in detail shortly, tread base 5 is obtained by amoulding and curing process from a rubber ribbon blank. Each lug 6 isobtained by the same mounding and curing process as a superimposition ofsaid rubber ribbon blank with a respective block or bar.

The manufacturing method described below allows tread base 5 to be ofsubstantially constant thickness.

The manufacturing method of tyre 1 is shown schematically in FIGS. 3 and3A to 3D.

In particular, casing 2 can be produced, in a conventional mode, on abuilding unit 101 and subsequently mounted onto a building drum 102 of aTyre Assembly Machine (TAM).

Upon building drum 102, tread belts 3 are applied onto casing 2.

In a conventional unit 103, tread rubber is mixed and extruded (orcalandered) into a strip 260.

In the same or in a separate unit 104, extruded (or calandered) strip260 is cut into green tread bars (or blocks or elements) 206 (FIG. 3A).Each bar 206 has an inner portion 209 corresponding to central portion 9of final lug 6 and an outer portion 208 corresponding to lateral portion8 of final lug 6.

Tread belts 3 and tread bars 206 can assembled together directly ontodrum 102, or they can be joined beforehand (i.e. “off-line”) andsubsequently placed on TAM and assembled with casing 2 and belts 3.

Bars 206 are lied upon belts 3 according to a selected arrangement andpredetermined relative angles (FIG. 3B).

Bars 206 can be joined to belts 3 according to several processes, e.g.by heating, by interposition of chemical compounds or by any other knownmethod for increasing rubber stitching and tackiness properties.

Subsequently, a tread ribbon 200 is would onto bars 206 lied upon drum102 (FIG. 3C). To this aim, tread rubber is mixed and extruded (orcalandered) in a green tread ribbon 200, for example in the same unit103 mentioned above. Ribbon 200 can be applied by a Ribbon TreadApplicator 106, already known per se in the relevant field.

Thereafter, the tyre to be built is removed from drum 102 (FIG. 3D) andplaced in a mould 105, wherein vulcanization (also using a press) takesplace according to conventional means and modes.

Strip 260 and/or ribbon 200 can also be manufactured in advance andstored in coils.

With reference to FIGS. 3A and 4, it will be appreciated that dimensionsand shape of strip 260 and bars 206 are not equal to the mould lug: theymay have a lower height and a wider base, eventually by defining almostsame volume. Similarly, sharp outermost edges can be avoided.

Still with reference to FIG. 3A, each bar 206 can have a constantcross-section, preferably with a cut angle α (and bare angle γ as well)common for the two end cross-sections. In this way, the cuttingoperation is simplified and rubber wastes are avoided.

Generally speaking, the manufacturing method according to the presentdisclosure is simplified with respect to the prior art. In particular,preparation and winding of tread ribbon 200 is easier than manufacturingand arranging tread blank below blocks according to the known methods.

It will be appreciated that the above method avoids the need forassembling tread blank and bars. In fact, application of bars 206 ontread plies 3 is done before application of ribbon 200.

As already pointed out, the method disclosed allows an importantreduction of excess rubber for the tread base with respect to the priorart.

Application of lubricants or other additives upon the semi-manufacturedtread before vulcanization to favour air flow and rubber detachment ispossible.

It will be appreciated that the method of the invention can be usedeither to build a new tyre or to regenerate the tread of a used one.

The present invention has been described so far with reference topreferred embodiments. It is intended that there may be otherembodiments which refer to the same inventive concept and fall withinthe scope of the following claims.

What is claimed is:
 1. A method of manufacturing a tyre tread (4), thetread (4) comprising a tread base (5) and a number of lugs (6), each ofwhich projects upwards from the tread base (5) and extends substantiallycrosswise to a travelling direction of the tyre, the method comprisingthe steps of: (a) proving a tyre casing (2) and tread belts (3); (b)arranging a plurality of rubber blocks (206) onto said tread belts (3),each of said rubber blocks (206) being configured to make a respectivelug (6) of the final tread (4); (c) winding a rubber ribbon (200) aboutsaid rubber blocks (206) and tread belts (3) arranged onto a drum (102)so as to shield said rubber blocks (206); (d) curing said rubber blocks(206) together with said wound ribbon (200) in a curing mould (105)negatively reproducing a desired pattern of the tread (4).
 2. The methodaccording to claim 1, wherein each of said blocks (206) has an innerwall (209) and an outer wall (208) substantially parallel one another.3. The method according to claim 1, wherein said rubber blocks (206) aremade of a different rubber compound or mixture with respect to saidribbon (200).
 4. The method according to claim 1, wherein said step (b)provides an operation of increasing rubber stitching properties of saidrubber blocks (206).
 5. The method according to claim 1, wherein saidstep (b) provides assembling said rubber blocks (206) onto said treadbelts (3) directly onto a construction drum (102).
 6. The methodaccording to claim 1, wherein, in said step (b), each of said rubberblocks (206) is arranged with an outer wall (208) located at a shoulderof the tread to be formed (4) and an inner wall (209) opposite the outerwall (208) and located in a central area of the tread (4) to be formed.7. The method according to claim 1, wherein, before said step (b), thereare provided further steps of: extruding (or calandering) a continuousrubber strip (260) having a cross-section corresponding to transversalsection of said rubber blocks (206); cutting rubber blocks (206) offsaid continuous rubber strip (260), preferably by an inclined crosswisecut.
 8. The method according to claim 1, which is a method ofregenerating used tyre treads.
 9. The method according to claim 1,wherein said tyre is an agricultural tyre (1), in particular anagricultural pneumatic tyre.
 10. The method according to claim 1,wherein, in said step (c), each of said rubber blocks (206) is arrangedso as to extend from a shoulder portion to a portion at a distance fromthe tyre centerline or from a shoulder portion to a portion beyond thetyre centerline.
 11. The method according to claim 1, wherein said step(c) is performed onto a TAM (Tyre Assembly Machine) drum, onto a TAMserver or offline.
 12. An apparatus for manufacturing a tyre (1), thetyre including a tread (4) comprising a tread base (5) and a number oflugs (6), each of which projects upwards from the tread base (5) andextends substantially crosswise to a travelling direction of the tyre,the apparatus comprising the following units arranged in an operativesequence: a rubber mixing and extruding unit (103) configured to outputan extruded or calandered ribbon (200) for making the tread base (5) andan extruded or calandered strip (260) for making rubber blocks (206),each of said rubber blocks (206) being configured to make a respectivelug (6) of the final tread (4); a drum unit (102) configured to allow atread casing (2), tread belts (3) and rubber blocks (206) be arrangedone upon the other; a ribbon tread applicator (106), configured to windthe ribbon (200) about said rubber blocks (206) so as to shield saidrubber blocks (206); a curing unit (105) of said rubber blocks (206) andwound ribbon (200), which curing unit (105) comprises a mould negativelyreproducing a desired pattern of the tread (4).